RFID systems are being utilized in a multitude of applications. One of the common areas for RFID implementation is product (for inventory or for sales) identification. The bar code scanner technology is being gradually replaced by RFID technology. In a simple application, a passive RFID transponder, commonly called a tag or a card, is placed on an object that is to be identified. A RFID reader is then used to obtain information from the tag. The reader typically has a transceiver to transmit and receive signals, the transceiver being powered by a power source. The tag also has a transceiver to receive a signal from the reader and to transmit a response back to the reader. The tag is generally passive and powered by the electromagnetic field induced by the reader.
When the reader is powered on, it is configured to generate an electromagnetic field from its antenna. When the reader and the tag are within close proximity of each other, the electromagnetic field generated by the reader is coupled to the tag. The tag uses this coupled energy to power its circuitry. The reader transmits an interrogating signal to the tag, and in response the tag transmits a signal back to the reader. The tag may be placed on an item and the response from the tag may be processed to identify the item.
RFID systems may use active tags which carry their own source of power such as a battery, or passive tags which contain no source of power and relies entirely on power radiated by the reader unit. A passive tag reader is configured to continuously or periodically search for the presence of passive tags in a vicinity of the reader by transmitting power to any tag that is present and activated by that power. A passive tag does not announce its presence unless activated by the reader.